1. Technical Field
The invention relates to zero voltage switching in power converters used in power conversion applications.
2. Related Art
A class D power converter may be used in switch-mode audio amplifiers. Class D signal amplifiers have been popular is recent years in audio applications due to advantages over legacy amplifiers. Less efficient amplifiers include Class A, Class B, Class AB, and Class H.
Class A amplifiers replicate and amplify the entire input signal, and therefore, are very linear. Consequently, they are typically inefficient. Even the most efficient theoretical Class A amplifier can operate at only about 50% efficiency. In general, for every watt of power output capability by a Class A amplifier, another watt is wasted as heat. Actual efficiency rates in Class A amplifiers are less than 10%.
Class B amplifiers in a piecewise linear manner replicate and amplify only half of the input signal. One of two amplifying elements is switched off alternately for every other half cycle. The most efficient theoretical Class B amplifier could operate at 78.5% efficiency. Actual efficiency rates with music are less than 30%. The Class AB amplifier balances the direct tradeoff between the higher efficiency of Class B amplifiers with the linearity of Class A amplifiers.
Class H amplifiers are similar to class A or AB amplifiers but have power supplies that switch between supply voltages to increase the output stage efficiency. The downside of these designs is complexity and the risk that power supply switching transients will not be fully removed from the output signal. Efficiencies can approach 50% with music.
Class D amplifiers are generally described as switching amplifiers because all of the power switching devices are either fully on or fully off. Class D amplifiers may use pulse width modulation (PWM) or another form of modulation. Because the input signal to a Class D amplifier is converted into a pulse train, power is not wasted as in other classes of amplifiers that amplify a signal in linear fashion. In addition to greater relative efficiency, relatively lower heat losses may result in use of relatively smaller heat sinks. The theoretical efficiency rate of Class D amplifiers is 100%. Actual efficiency rates of Class D amplifiers may approach 95%.
Class-D half-bridge power converters used in switch-mode audio amplifiers, as well as other DC to AC power conversion applications may achieve higher efficiencies and lower electromagnetic interferences (EMI) by operating in a zero voltage switching (ZVS) mode of operation. With ZVS mode, during operation, switch devices in a switching stage of the power converter are activated at zero crossings of their main terminal voltage to minimize turn on losses. An amount of time is required by the switch devices to turn off and on. The overlap between these transitions can be referred to as dead-time. A minimum amount of dead-time is needed to avoid having both switch devices closed at the same time. If both switch devices are closed at the same time, potentially destructive shoot-through current that travels directly from supply rail to rail may result. Too much dead-time may result in distortion in the output signal.